This order contains six English genera, Epilobium Œnothera, Ludwigia, Circaea, Myriophyllum, and Hippuris.

The instructive differences which exist between the different species of Epilobium have already been referred to in the introductory chapter. Œnothera biennis is really a North American plant, though now naturalized in some parts of England. As its name denotes (Evening Primrose) it is a yellow night flower; it secretes honey, and is probably fertilised by moths, though it remains open by day, and is also visited by bees. Ludwigia contains a single species, L. palustris - a minute marsh plant, hitherto found in very few localities in Britain, though it ranges over Central Europe, Asia, and North America. The genus Circaea contains two species, C. alpina and C. lutetiana, the Enchanter's Nightshade. This species has two stamens, and as the flower is small, any insect of moderate size would probably touch both them and the pistil; most likely, however, coming in contact with the stigma first, as it projects rather beyond the anthers.